Since we want to find Voltage across the 1kΩ resistor using Thevenin's theorem, I removed the resistor from the circuit.
Now to find the voltage across the Open terminals I would have to simplify the circuit. Where I am stuck is how I can simplify the rest of the circuit so that I have Voltage and R_th.
The current sources are baffling me, I dont know how to simplify those.

I tried using Mesh analysis because i have 2 of the 3 currents but I dont know how to use those currents to find a voltage across the terminals.

Staff: Mentor

With your calculated I3 you should be able to find the voltage drop across the 6kΩ resistor which is in series with the 12V supply. The 2mA current source forces the potential drop across the 2kΩ resistor. Add a touch of KVL and you have your Thevenin voltage.